The need to reduce the cost and size of electronic equipment has caused a need for smaller signal filtering elements. Thin Film Bulk Acoustic Resonators (FBARs) and Stacked Thin Film Bulk Wave Acoustic Resonators (SBARs) represent one class of filter element with potential for meeting these needs. These filters can collectively be referred to as FBARs. A FBAR is constructed from acoustic resonators using bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. Typically, a FBAR includes a layer of PZ material sandwiched between two metal electrodes. The combination PZ material and electrodes are suspended in air by supporting the combination around its parameter.
When an electric field is created between the two electrodes, the PZ material converts some of the electrical energy into mechanical energy in the form of acoustic waves. The acoustic waves propagate in the same direction as the electric field and reflect off the electrode-air interface at some frequency including at a resonance frequency. At the resonance frequency, the device can be used as an electronic resonator. Multiple FBARs can be combined such that each are elements in RF filters.
Ideally, the resonant energy is “trapped” in this longitudinal mode. However, in practice, parasitic lateral modes (and other non-longitudinal modes) exist. These modes act to take out energy from the desired longitudinal mode. Furthermore, some of these parasitic modes are quite lossy as they are impacted by the edges of the resonator. This loss is manifested by both sharp “high-Q” modes (which look like “rattles” in the Q circle) and by “low-Q” lossy modes (a more broad-band loss).
For these and other reasons, a need exists for the present invention.